Method of producing niobium metal



May 24, 1960 H. A. WILHELM ETAL 2,937,939

METHOD OF PRODUCING NIOBIUM METAL Filed Nov. 10, 1958 Pefcefli excess carton INVENTORS jlarlqyfl [ail/elm Ernest fgyer Safer/e115 BY Jllorzzgy I I stoichiometric quantity.

2,937,939 METHOD or PRODUCING NIOBIUM METAL Harley A. Wilhelm and Ernest Roger Stevens, Ames, Iowa, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Nov. 10,'1958,-Ser. No. 773,118

Claims. (Cl. 75-84) V This inventiondeals with a method of producing niobium metal and in particular with a method based .on

the reduction of niobium pentoxidewith carbon.

The reduction reaction of niobium pentoxide with carbon proceeds predominantly according to the following equation: a a i Originally the process used to be carried out with an amount of carbon excessive overthat stoichiometrically required in order to secure complete reduction. However, it was found that the excess carbon reacted with the niobium metal produced and niobium carbide was formed and that the final product was hard and lacked sufiicient ductility to make it workable; this hardness. increased gradually with increasing carbide content.

Itwas then tried to carry out the reaction with a formed from the additive must be either volatile under' operating conditions or else it must be a material that does not have an appreciably adverse effect on the ductile properties of the metal or alloy formed. r

Titanium metal, titanium carbide and titanium-oxides, when properly used, have all three of the characteristics just enumerated, and all three are satisfactory additives for the process of this invention. If titanium carbide is used, it is advisable to include the carbon content of the carbide in the determination of the carbon amount required for the reduction, because the carbide carbon will also take part in the reduction of the niobium pentoxide.

Titanium dioxide is the preferred additive, because it is considerably less expensive than the other suitablema- V terials. I

The function of the titanium and titanium compounds I in the reduction reaction is not exactly known. i Howquantity of carbon less than the stoichiometric amount.

This, however, resulted in even more unfavorable results, because some unreduced niobium oxide then reof amount of carbon added to the reaction mass and hardness of the niobium metal obtained.

I It. is rather difiicult to adjust the amount of carbonto exactly the stoichiometric quantity, this especially because niobium oxides vary as to their content of impurities. I

It is an object of this invention to provide a process I for the production of niobium metal from niobium pentoxide and carbon by which a metal of a high degree of I ductility is obtained. 7

It is another object of this invention to providea process for the production of niobium metal from niobium pentoxide and carbon by which a high degree of ductility is obtained no matter whether the carbon is added in an excessive or in a deficient amount as to the These objects are accomplished by adding a titaniumcontainingmaterial to the mixture of niobium pentoxide and carbon, heating the reaction components while under reduced pressure to a temperature above about 1300 C. but below the melting point of niobium metal whereby a ductile niobium metal sponge substantially free from oxygen and from carbon is obtained.

'It was found that, in order to" be suitable as an additive for-the niobium pentoxide reduction with carbon, a substance has to have the following characteristics: (1) it, or one of its'reaction products, has to be ableto re-. eact with any residualoxygen-in the metal; (2) any oxide formed from the additive or its reaction products during reaction has to be volatile .under the operating conditions; and (3) any product other than the oxide' ever, it is believed that titanium oxide, for instance, reacts with an excess of carbon, forming carbon oxide, titanium metal and titanium oxides that are volatile at the reaction temperature and the reduced pressure; the titanium metal forms alloys with the niobium, and a ductile titanium-niobium alloy is then obtained. On the otherhand, in a carbon-deficient mixturethe titanium oxide reacts with the carbon, this preferentially to the reaction of niobium oxide with carbon, to form, metallic titanium or a lower oxide of titanium which then reacts with unreacted residual niobium oxide to form niobium metal and titanium oxide; the latter volatilizes under the conditions used for the process of this invention. As has been mentioned before, there is no proof for the correctness of these explanations.

The reduction reaction of niobium pentoxide car- Subatmospheric pressure was used for carrying out the process of this invention, and an initial vacuum corre-. sponding to a pressure of about one micron was pre ferred. Evacuation was continued during the entire time of the process but itwas not enough to remove the car'- bon oxide and/or carbon dioxide at the rate they were formed. Consequently the pressure during the reaction gradually increased up to about 900 microns. Heating, too, has to be continued during this phase of the reaction and until the pressuredrops back to the initial pressure of about one micron which indicates that substantially no more gas is developed and the reaction has mole to an excess of 0.6 mole per one mole of niobium' pentoxide. The titanium, in the form of the metal or of the compound, can be added in a quantity ranging from 0.05 to 0.4 mole per one mole of niobium pentoxide. The ratio for molar carbon deviatiommolar titanium addition (as dioxide), again for one mole of niobium.

Patented May 24, :1960

available pentoxide, can range from -0.5 to +1.5, the range from 0.5 to being applicable for operation with carbon deficiency and the range from O to +1.5 being pertinent to operation with excess carbon. (The expression molar carbon deviation covers both positiveexcessand negativedeficiencywith regard to the stoiohiometric amount of carbon (assuming that only carbon oxide and no dioxide is formed), which is 60.05 grams per 266 grams of niobium pentoxide.)

Various methods and types of equipment known to those skilled in the art can be used for carrying out the process of this invention. In the experiments that led to this invention, predetermined amounts of powdered niobium pentoxide, carbon and titanium material were mixed, wetted with acetone to obtain a pasty consistency and the paste was then mixed once more. Thereafter the paste was dried in an oven at about 100 C.; the dried product was ground and dry-mixed in a ball mill. Acetone was again added until a paste was obtained. The paste was then molded in a glass pipe, and while in the glass mold it was dried at about 100 C. The mass shrank, and the molded cylinder of reaction mass thus could be easily removed from the glass pipe; it was transferred into a tantalum crucible.

The tantalum crucible, uncovered, with the charge was then placed inside a graphite heater which was provided with a chimney at its top. This chimney not only allowed the escape of the gaseous reaction products, but it also provided for a path through which temperature measurements could be made with an optical pyrometer. The graphite heater was placed into a cylindrical Pyrex jar which was considerably wider than the heater and which was thermally insulated therefrom by filling the interspacc with carbon black.

This assembly was placed on a perforated stand which was supported by a water-cooled base. A quartz tube which had a cover on its top was placed over this assembly and seated on this base. The cover had a window in its center which was aligned with the chimney of the heater so that observation was possible. The base was connected with a vacuum system. For evacuation, a mechanical pump and an oil diffusion pump were used. The quartz tube was surrounded by induction heating coils.

The entire system was evacuated very slowly in order to keep the carbon insulation from blowing out and also to keep the charge intact. The pressure was reduced to about 02x10" mm. Hg, and at this point induction heating was started.

Pressure readings were made with a thermocouple vacuum gauge. Both temperature and pressure were read each minute during the run. The predetermined maximum temperature was maintained until gas evolution was negligibly low and the pressure had receded to the initial value of about 0.2 l0 mm. Hg. The heat was then turned off, and the product was allowed to cool to room temperature while the vacuum was maintained. The product of the reduction was obtained in the form of a cylindrical metallic sponge.

'The sponge may be compacted, for instance by melting, by pressing and sintering, or by arc-melting preferably under one-half an atmosphere of helium whereby a metal button is obtained; arc-melting was the preferred method. Each button was cut into sections about /8 inch thick. The sections were examined for hardness and workability by cold-rolling, and they also were analyzed for titanium and oxygen content by spectrographic methods and for carbon by chemical methods.

In the following, two examples were given to illustrate the process of this invention; the details given in these examples are not intended to limit the scope of the invention.

Example I Niobium pentoxide in a quantity of 26.6 grams, 2.48

grams of titanium dioxide and 6.1 86 grams of carbon were mixed as described above. Of the mixture thus obtained, 34.4 grams were charged into the tantalum crucible for reduction. This carbon content was excessive over the stoichiometric amount by about 0.15 mole per mole of Nb O The tantalum crucible was heated in the assembly described above after the pressure had been reduced to practically zero. Within 13 minutes the pressure increased to 600 microns and the temperature at that point had reached 1320 C. Within the next 10 minutes the temperature reached 1685 C.; during this entire 10-minute period the pressure remained constant at 600 microns. Thereafter the temperature reached 1855 C. within 6 minutes; during this period the pressure gradually went down to -l70 microns. Then heating was still continued until the temperature had reached 2060 C.; this took 20 minutes. By this time the pressure had receded to 27 microns. This temperature of 2060 C. was maintained for 32 minutes; after this time the pressure was back to about zero. Then the heat was turned off, and the mass was allowed to cool while the vacuum was maintained.

The metal output after arc-melting was 17.1 grams which corresponds to a yield of 92%. The hardness was determined as 39 Rockwell, scale A. The niobium metal obtained had an oxygen content of 480 p.p.m., a carbon content of 447 p.p.m. and a titanium content of slightly above 0.4%. The arc-melted niobium metal was cold rolled without ditficulty into a sheet of approximately 6 mils thickness, and only very minor edge-roughening occurred.

Example II Again 26.6 grams of niobium pentoxide were used; they were mixed with 1.6 grams of titanium dioxide and 5.76 grams of carbon. (The niobium oxide was found to contain 1.44% by weight of volatile ingredients which were completely removed at 1000 C. Considering this content of volatile ingredients, the carbon added actually was 0.487 mole or deficient by 0.13 mole for one mole of niobium pentoxide.) Of this mixture 33.5 grams were introduced, as the charge, into the tantalum crucible.

The equipment containing the charge was evacuated as in Example I until the pressure was practically zero, and then the mixture was slowly heated. A maximum pressure of 400 microns was obtained at 1150 C. after 16 minutes of heating. This pressure remained constant for 9 minutes when the temperature was 1400 C. Heating was still continued until the temperature was 1850" C.; this took about 30 minutes, and during this period the pressure continually decreased to about 10 microns. The temperature was held and the pressure remained constant for the next 6 minutes whereupon the temperature was still furthermore raised to 2000 C. within 26 minutes. By then the pressure had reached a low of one micron. At this point heating was discontinued, and the mass was allowed to cool while the vacuum was maintained.

The arc-melted metal weighed 16.55 grams; this corresponds to a yield of 89%. It had a hardness of 48, Rockwell A, and it could be cold-rolled as easily as the metal obtained in Example 1. Analysis showed an oxygen content of 230 p.p.m., a carbon content of 399 p.p.m. and a titanium content of approximately 1000 p.p.m.

Niobium sponge can be used as such for the preparation of niobium-containing alloys with other metals. One of the many uses of niobium metal in the form of sheets is as a getter in high-vacuum tubes for radios, etc. Another use of rolled niobium sheets is as a barrier between uranium-base metals and zirconium to prevent the formation of lower-melting alloys.

It will be understood that this invention is not to be limited to the details given herein, but that it may be modified within the scope of the appended claims.

What is claimed is:

1. A process of producing a ductile niobium metal, comprising mixing niobium pentoxide, carbon and a titanium-containing material selected from the group consisting of titanium metal, titanium carbide, titanium oxide and any mixture thereof, the quantity of said titaniumcontaining material corresponding to between 0.05 and 0.4 atom of titanium per mole of niobium pentoxide; heating the mixture under subatmospheric pressure to a temperature of above about 1300 C. but below the melting point of niobium metal, whereby the gaseous reaction products formed are removed and substantially carbonand oxygen-free niobium metal sponge is obtained; and cooling the niobium metal sponge under reduced pressure.

2. A process of producing a ductile niobium metal, comprising mixing niobium pentoxide, carbon and a titanium-containing material selected from the group consisting of titanium metal, titanium carbide, titanium oxide and any mixture thereof, the quantity of said titaniumcontaining material corresponding to between 0.05 and 0.4 atom of titanium per mole of niobium pentoxide; heating the mixture under subatmospheric pressure to a temperature of above about 1300 C. but below the melting point of niobium metal, whereby the gaseous reaction products formed are removed and substantially carbonand oxygen-free niobium metal sponge is obtained; cooling the niobium metal sponge under reduced pressure; and compacting the niobium metal sponge.

3. The process of claim 2 wherein the titanium-containing material is titanium dioxide.

4. The process of claim 2 wherein the titanium-containing material is titanium oxide of the formula Ti O 5. The process of claim 2 wherein the mixture is heated to a temperature of between 1600 and 2200 C.

6. The process of claim 2 wherein the pressure is reduced to about 1 micron Hg.

7. The process of claim 2 wherein at least part of the carbon is added in the form of niobium carbide.

8. The process of claim 2 wherein at least part of I the titanium-containing material and at least part of the carbon are added in the form of titanium carbide.

9. The process of claim 2 wherein at least part of the carbon and at least part of the titanium are added in the form of a mixture of niobium carbide and titanium carbide.

10. The process of claim 1 wherein the mixture contains the carbon in an amount ranging from a deficiency of 0.1 mole to an excess of 0.6 mole with reference to the stoichiometric amount per 1 mode of niobium pentoxide, and wherein the molar ratio of a carbon deficiency to amount of titanium, as dioxide, is within the range of from -0.5 to zero and the molar ratio of a carbon excess to amount of titanium is within the range of from zero to +1.5.

References Cited in the file of this patent UNITED STATES PATENTS 908,682 Lederer Jan. 5, 1909 992,422 Hufiard May 16, 1911 OTHER REFERENCES AEC Document BMI-1003, May 23, 1955, pages 14 and 15. 

1. A PROCESS OF PRODUCING A DUCTILE NIOBIUM METAL, COMPRISING MIXING NIOBIUM PENTOXIDE, CARBON AND A TITANIUM-CONTAINING MATERIAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM METAL, TITANIUM CARBIDE, TITANIUM OXIDE AND ANY MIXTURE THEREOF, THE QUANTITY OF SAID TITANIUMCONTAINING MATERIAL CORRESPONDING TO BETWEEN 0.05 AND 0.4 ATOM OF TITANIUM PER MOLE OF NIOBIUM PENTOXIDE, HEATING THE MIXTURE UNDER SUBATMOSPHERIC PRESSURE TO A TEMPERATURE OF ABOVE ABOUT 1300*C. BUT BELOW THE MELTING POINT OF NIOBIUM METAL, WHEREBY THE GASEOUS REACTION PRODUCTS FORMED ARE REMOVED AND SUBSTANTIALLY CARBON AND OXYGEN-FREE NIOBIUM METAL SPONGE IS OBTAINED, AND COOLING THE NIOBIUM METAL SPONGE UNDER REDUCED PRESSURE. 